A vented rainscreen is an increasingly common exterior detail on exterior walls. Instead of attaching siding directly to the sheathing, builders add an air space behind the siding, which helps carry away moisture and prolongs the life of the siding.
Builders typically use vertical furring strips to create the gap, but there also are commercial products like Home Slicker, a plastic mesh, that are designed to do the same thing.
A rainscreen certainly is an extra step in construction, but many builders think it’s well worth the effort. So, Eric Mikkelsen asks, are vented rainscreens equally valuable on a roof?
“I’ve seen lots of info about rainscreen walls,” he writes in a Q&A post. “What about roofs? I’m planning on using a standing seam metal roof, 10/12 pitch. Should I be considering rainscreen details for this?”
Although Mikkelsen’s original post dates from 2012, the question is still sparking interest among GBA readers.
Is the attic ventilated?
James Morgan’s first question is whether the attic is ventilated, which would leave the underside of the roof sheathing open for inspection. If the attic is ventilated, Morgan says a rainscreen detail on the roof is unnecessary.
The roof, Mikkelsen says, is insulated with spray polyurethane foam applied on the underside of the sheathing, meaning the roof is not ventilated.
GBA editor Martin Holladay notes that a common approach for builders worried about ice dams — admittedly a concern that rarely plagues homes with metal roofing — is to apply 2x4s on the flat on top of the roof sheathing, with one 2×4 over each rafter or truss. Then a second layer of roof sheathing goes on, followed by roofing underlayment and the roofing. The 2x4s will create a 1 1/2-inch air gap between the two layers of sheathing.…